1. Field of the Invention
The invention is directed to an apparatus for abrading the surface of a workpiece by providing a compound motion to produce an irregular surface pattern.
2. Background Art
The final stages of producing a vehicle brake drum are directed to providing a smooth irregular pattern on the inside of the brake drum using a three-step process. In the first step a single point cutting tool for roughing is applied to bore the inner diameter of the work surface as the drum is rotated. In the second step a single point cutting tool for semi-finishing is applied to the roughed work surface. In both steps, as the tool is fed along the work surface, feed lines form in a spiral pattern. These feed lines are objectionable for commercial applications because a brake pad applied to a surface having this pattern will engage the spiral grooves of the feed lines and be forced to move laterally when the brake is applied causing brake slap. This condition is unacceptable because a lateral force on the brake shoe prematurely wears the material on the brake shoe and also a significant lateral force on the brake shoe may damage the associated brake hardware. For these reasons, these spiral grooves must be eliminated.
Therefore, the third step of the process involves using an abrasive paper applied to the drum surface under pressure by a device that looks similar to a brake drum shoe. During this process a kerosene based coolant is run over the abrasive paper and workpiece. The step involves using a specially built machine that is dedicated to this process as well as additional equipment for cleaning and swarf removal.
A disadvantage of this three-step process exists because one machine is used for the first two steps and a second machine is used for the third step. As a result, the brake drums are transferred from one machine to another and are subjected to runout and imbalance conditions which is referred to as transfer error and which occurs when parts are moved from one machine to another and as a result are chucked on different surfaces of each machine. If the roughing/semi-finishing operations and the surface finish operation could be performed on the same machine, then an improvement of overall part quality as well as a reduction in the number of required machine tools, cycle time and cost would be possible.
U.S. Pat. No. 2,606,410 entitled "Device for Superfinishing Machined Surfaces" discloses in FIG. 6 a superfinishing abrasive stone 13 mounted upon a leaf spring 12 which is urged against the inner surface of a rotating cylinder 44. The abrasive stone 13 is attached to a snug 14 which may move within a grooved bar 11 to contact and abrade in a reciprocating fashion the inner surface of member 44. The device disclosed in this patent is completely dedicated to finishing the machined surface and a boring step using a separate machine must precede this finishing step. Therefore, this operation requires a two-step process which introduces transfer error. Furthermore, the rotation of member 44, coupled with the oscillation of the abrasive stone 13, produces a repeating surface pattern which is not an irregular surface pattern desired for brake drums. On the contrary, such a repeating pattern tends to be sinusoidal and this pattern may establish undesirable harmonic vibration when a brake pad is pressed against its surface.
U.S. Pat. No. 2,244,806 entitled "Honing Apparatus" and U.S. Pat. No. 2,276,611 entitled "Honing Apparatus" are both directed to devices for honing the inner wall of a rotating cylinder using an abrading stone. While the '611 patent utilizes a rotating cam 17 to provide the reciprocating motion, the '806 patent utilizes a reversing valve 27 which directs pneumatic pressure to chambers on each side of a piston to reciprocate the abrading stone. Just as was the case with the 410 patent, each of these two devices is associated with a machine dedicated to the single function of honing and therefore it is necessary to transfer the workpiece for honing from another machine that machined the bore, thereby introducing the previously-discussed transfer error. Additionally, the single reciprocating motion imparted to the abrasive stone by each of these devices against the rotating workpiece produces a regular finish which, as previously discussed, is undesirable.
An apparatus is desired for imparting to a workpiece an irregular surface pattern which is preferred for the contact surface of, for example, vehicle brake drums.